Programming

I just spent an hour tracking down this exact bug because a function declaration wasn't in the header file where I thought it was. It then led to bizarre behavior.

f.c

Code:

#include <stdio.h>

void f(long long a, long b)
{
  printf("%llx, %lx\n", a, b);
}

main.c

Code:

#include <stdio.h>
int main()
{
  long x = 0x11111111;
  long y = 0x22222222;
  long z = 0x3333333333333333ll; // lower-case LL at the end

  f(x, y);
  f(x, z);
  return 0;
}

What's output may surprise you. You'd expect to see
11111111, 22222222
11111111, 33333333

instead, you get
2222222211111111, 0
3333333311111111, 33333333

To fix the code and get the implicit casting behavior, include a function prototype in main.c. Luckily, I was passing in two longs, and caught the bug from a divide-by-zero message. I'd hate to think how confused I would have gotten if I was passing in two long long's.

I recommend always compiling with highest level of warning.

With gcc using "-Wall -Werror" does this.